High styrene and high vinyl SSBR is difficult to produce due to the kinetics of copolymerization. Typically, polar agents known as randomizers are added to the polymerization system in order to achieve random styrene incorporation.
The use of certain randomizers can result in high vinyl SSBR having a low block styrene content (>6 successive units of styrene) below 10%. Long block styrene can worsen hysteresis as reported, for example, by S. Futamura and G. Day who observed a worsening by about 18% of the tan delta at 60° C. when increasing block styrene content from 2 to about 7% (Kautschuk Gummi Kunststoffe, 1987, 40, No. 1, 39-43) in a carbon black-filled compound. By contrast, incorporation of small styrene blocks can result in improved abrasion and tensile strengths, particularly in silica compounds, as reported by I. Hattori et al. (143rd Meeting of the Rubber Division of the ACS, Spring 1993, paper 22).
Potassium 3,7-dimethyl-3-octylate is described in U.S. Pat. No. 6,521,712 for the preparation of random low vinyl soft blocks in block copolymers. Likewise, U.S. Pat. No. 6,197,889 describes the use of potassium 3,7-dimethyl-3-octylate as randomizer. In both patents, the molecular weight of the resulting polymer is very low (range 3000-200,000 g/mol).
In U.S. Pat. No. 3,294,768, the use of sodium and potassium alcoholates as randomizer for low vinyl SSBR is reported. In U.S. Pat. No. 3,787,377, sodium and potassium tert-amylate and mentholate are described in the context of continuous anionic polymerization, at a temperature from 110 to 125° C. U.S. Pat. No. 5,916,962 describes a conjugated rubber composition, which shows a broad molecular weight distribution of 1.7 or more after coupling with silicon tetrachloride.
In certain applications, it would be desirable to achieve a high styrene high vinyl SSBR having a defined incorporation of styrene as blocks of more than 4 consecutive styrene units with a narrow molecular weight distribution.